It's Hue!

We tend to be more sensitive to changes in hue than
to changes in
brightness,
and more sensitive to changes in brightness
than to changes in
saturation.

Imagine if traffic lights had a dark red,
a medium red, and a bright red color instead of the different hues
of green, yellow, and red that are commonly used.
As you approached this new traffic light you might be confused
wondering whether the light was medium red or dark red.
However, when you approach a red/green/yellow traffic light it's
easy to tell whether to go or stop.